I don't really get what you are asking, so I will just comment on random bits that you said.
I don't think it feels like a hack; this should work just fine.
Where did z depth buffers come into the picture?
I'm asking about the best way to implement a post processing system into an existing engine. Check http://en.wikipedia.org/wiki/Video_post-processing
if you don't know what it is.
The depth buffer is important in many post processing effects, so I need to have an easy, non expensive way to access it. Currently I have to render it to a whole nother FBO and send that in to my post processing shaders in order to use it, and that may not be the best way. However this is the way I will have to do it once I implement deferred rendering, so I may want to just keep it this way.